1. Field
This disclosure relates to the field of deriving electrical power from a heat source. More specifically, this disclosure relates to extracting both thermal and photonic energy from a heat source, especially from a thermal flow.
2. Description of the Related Art
A common thermoelectric (TE) generator is illustrated in FIG. 1. TE generators convert heat into electric current or voltage. Holes and electrons are produced in the P-type and the N-type TE elements, respectively, as a result of thermal excitation and, preferably, doping of the constituent materials. The TE elements have one end (the hot end) contacting a heat exchanger that is at a high temperature and have an opposite end (the cold end) contacting a heat exchanger or heat sink that is at a cold temperature. The gradient in temperature that is established in the TE elements results in a net flow of electrical carriers from the hot end to the cold end, or the establishment of an electrical potential to oppose such a current flow, because of a thermoelectric effect (e.g., the Seebeck effect).
The TE elements are typically arranged in pairs, or TE couples, that comprise a P-type element and an N-type element. The hot ends of these two elements are joined together by electrical contacts to a common metal interconnect. These contacts serve as a source of the majority carriers (holes in the P-type element and electrons in the N-type element) that carry the net thermoelectric current. On their cold ends, the P-type element and the N-type element of a given TE couple are electrically connected to each other only through an external circuit. Higher thermoelectric voltage may be obtained by electrically connecting multiple TE couples in series, as illustrated in FIG. 1. Higher thermoelectric current may be obtained by electrically connecting multiple TE couples in parallel.
The size of that electrical current depends on both the temperature gradient and on the Seebeck coefficient and the electrical conductivity of the materials comprising the TE element. The temperature gradient, −VT, also can result in a heat flow (thermal current) that depends on the thermal conductivity of the materials. For highest TE efficiency, this heat flow is not desirable since it can reduce the strength of the temperature gradient. Thus, preferred TE materials have high electrical conductivity s, a large Seebeck coefficient S and low thermal conductivity b. These relationships are described in the equations below:Electrical current: J(r)=s(F−(1/q)VEf)+sS(−VT)Heat flow: JQ(r)=sST(F−(1/q)VEf)+b(−VT)where (1/q)VEf is the additional carrier injection and sS(−VT) is the thermoelectric effect. The heat flow through a TE element provided by the thermal conductivity of that TE element is described by b(−VT). Note that additional electrical current through a TE element can result from a local electrical field F existing between the hot and cold ends of that element and also from a gradient in the density of electrical carriers, −(1/q)VEf, existing between the hot and cold ends of that element.
There are some alternative generators of electrical power that make use of the temperature difference between a hot end and a cold end. One such prior art generator is described in an article by G. Span, et al. (“Thermoelectric power generation using large area pn-junctions”, Proceedings of 2005 European Conference on Thermoelectrics, 2005) and in a patent issued to G. Span (U.S. Pat. No. 6,762,484 B2). This generator comprises a P-type layer and an N-type layer that are electrically in contact and that form a PN junction. One end of the PN junction is at a hot temperature and an opposite end of the PN junction is at a cold temperature. The PN junction is formed over the entire length of the P-type and N-type layers. Carriers (electrons and holes) are thermally generated in the depletion region of the PN junction. The built-in potential of the PN junction separates the electron and holes, injecting the electrons into the N-type layer and the holes into the P-type layer.
Overall, there is net generation at the hot side and net recombination of carriers at the cold side. Thus, there is a net flow of injected carriers from the hot side to the cold side. In general, the P-layer and the N-layer of this prior generator both are made from the same material type (e.g., both comprising Si or both comprising SiGe), although those layers have different doping (e.g., p-doped and n-doped, respectively). However, the composition of that material (of both the P-layer and the N-layer) may vary along the length of the PN junction from the hot side to the cold side. For example, the material of both the P-type and the N-type layers can change from being Si at the hot side to being SiGe at the cold side. Use of varying composition material is described in a paper by M. Wagner, et al., “Thermoelectric power generation using large-area Si/SiGe pn-junctions with varying Ge-content,” Proceedings of 2006 International Conference on Thermoelectrics (2006).
These prior art generators contain PN homojunctions in which the P-layer and the N-layer are made from the same material type. In contrast, the presently disclosed enhanced power generation system contains heterojunctions in which the P-type element, the N-type element and a carrier generating photon converter preferably are made from different material types. In further contrast with the prior PN junction generator, for which the PN junction extends fully from the hot-side to the cold-side, the carrier generating photon converter presently disclosed is located primarily at the hot end and preferably does not extend to the cold end.
Another type of prior art generator is described in a paper by Y. Kucherov, P. Hagelstein, et al., (Importance of barrier layers in thermal diodes for energy conversion,” Journal of Applied Physics, v. 97, p. 094902, 2005). This prior generator contains a heavily doped N-type emitter located at the hot end of the device. The emitter thermally generates electrical charge carriers (e.g., electrons) and injects those carriers into the solid gap region, which comprises a more lightly doped N-type material. A P-type electronic energy barrier (separation layer) prevents carriers that are thermally generated in the gap region from being injected back into the emitter. Thus, there is net flow of the carriers from the hot side to the cold side. Although there may be some carrier flow from the hot end of the gap region to the cold end of the gap region because of the temperature gradient along that gap region, much of the net carrier flow is produced because of the carrier injection from the emitter. In general, the emitter, barrier and gap region are made from the same material type (e.g., all made from InSb or all made from HgCdTe) although these regions have different doping.
These prior art generators involve thermal generation of electrical carriers outside a TE region and injection of those carriers, as majority carriers, into the TE region. In contrast, the presently disclosed photon enhanced power generation system provides a carrier generating photon converter in which at least some of those electrical carriers are produced as a result of photon absorption.
A Thermo-Photovoltaic (TPV) generator is illustrated in FIG. 2. An article by Zenker, et al. (“Efficiency and power density potential of combustion-driven thermophotovoltaic systems using GaSb photovoltaic cells,” IEEE Transactions on Electron Devices, vol. 48, n. 2, February 2001, pp. 367-376) discusses the anticipated performance and the construction of TPV generators. Prior TPV generators make use of an input flux of heat to raise and set the temperature of a photon emitter. Some of the radiation emitted by the photon emitter is then absorbed by a photovoltaic (PV) cell and converted to electricity. The excess energy of those photons whose energy is greater than the bandgap of the PV-cell material is converted to waste heat that must be removed or else it will degrade the performance of the PV cell. In many prior TPV generators, the conversion efficiency is increased by selecting PV cell material that has a larger bandgap rather than a smaller bandgap. This selection, however, reduces the power that is generated since a smaller percentage of the incident photons are absorbed (because only those photons have energy greater than that higher bandgap). In general, to achieve enhanced conversion efficiency, it is preferable to maximize the difference between the temperature of the photon emitter and the temperature of the PV cell. This larger temperature difference reduces the amount of black body radiation of photons by the PV cell.
An article by A. Shakouri (“Thermoelectric, thermionic, and thermophotovoltaic energy conversion,” Proceedings of the 2005 International Conference on Thermoelectrics, 2005) compares the relative merits of conventional thermoelectric and conventional thermophotovoltaic conversion processes. In this article, the author states “It seems that working with different energy carriers (electrons, photons, etc) and with reservoirs with different internal degrees of freedom may provide another opportunity to engineer the efficiency of the heat engines and to approach the entropy limit (2nd law of thermodynamics) more easily.” The author, however, does not describe any method to make use of both photons and electrons in generating electrical power.
Many of the prior attempts at TE power generation have considered the case in which heat is transferred by conduction from a constant-temperature region through a heat exchanger to the hot end of the TE elements. In the case of conductive heat transfer, in contrast to convective heat transfer, the benefit of absorbing photons is not as great. Rather, most designs of conventional TE generators attempt to minimize the temperature difference between the heat source (e.g., the combustion products) and the hot end of the TE elements while still extracting enough heat from the heat source.
When heat is being extracted by convective heat transfer from a flowing gas or fluid by a heat exchanger, there typically is a thermal boundary layer between the central portion of the flow and the surface of the heat exchanger. Thus, there typically is a local temperature drop, δT, between the central portion of the flow and the surface of the heat exchanger facing the flowing gas or fluid. This temperature drop is in addition to the temperature drop between the inlet end and the outlet end of the flow. Even the case involving multiple thermally isolated heat exchangers (described later) would have this local temperature drop, δT. As a result, there would be some net photon flux toward the hot ends of the TE elements. An article by K. Matsubara (“Development of a high efficient thermoelectric stack for a waste exhaust heat recovery of vehicles,” Proceedings 21st International Conference on Thermoelectrics, pp. 418-423, 2002) describes a demonstration of a thermoelectric generation system for recovering energy from the waste exhaust heat of an automobile engine. Measurements were made of the exhaust gas temperature at various points between the inlet and outlet ends of the heat exchanger. Additional measurements were made of the temperatures of the hot ends of the TE elements corresponding to those points. At any point along the flow direction, the article reports a difference of approximately 100K between the temperature of the exhaust gas and the temperature of the heat exchanger. The article also reports an additional temperature difference between the surface of the heat exchanger and the hot end of the TE elements. The enhanced TE generator disclosed herein makes use of these temperature differences to increase the total electrical power that is produced.
Utility
Motor vehicles such as cars and trucks are based on internal combustion engines. In most motor vehicles, a relatively small percentage (<25%) of the energy generated as a result of combusting the fuel in the engine is used to actually move the vehicle. Some of the energy generated (typically 1-10%) provides another useful function, being used to generate electricity. Typically, a belt driven electrical alternator is used to generate that electricity. Electricity has many uses in a motor vehicle. For example, electricity may be used to power accessories such as radios, fans, heaters and air conditioners, to power lights and electronic controllers and computers, to activate switches and valves, to move power windows and wiper blades, to adjust the suspension elements, to activate and control brakes and pumps, etc. A large percentage of the energy generated by the fuel combustion process (as much as 70%) is in the form of waste heat. Thermoelectric generators provide one means to recover the energy in this waste heat and convert that energy into electricity. Approximately 40% of the energy from the fuel combustion is carried in the hot exhaust gas. If a thermoelectric generator that can access this hot exhaust gas has a power conversion efficiency of only 10%, that generator can redirect 4% of the energy consumed into additional electrical power. This would represent a significant fraction of the electricity used in a vehicle.
The power generators disclosed herein could be relevant to any system that recovers waste heat from a flow of hot gas or fluid. This kind of waste heat recovery also is being considered for hot water heaters, gas turbines, industrial furnaces, and fuel-burning electrical power plants. While the generators described are capable of generating electric power from nearly any type of heat source, they are especially useful in extracting power from a thermal flow (a flow of hot gas or fluid).